Newly drilled petroleum reservoirs often produce oil without water. As they produce over time these reservoirs naturally decline in pressure or start making water. In either case the reservoirs typically require artificial lift to continue producing. Sucker rod pumps deployed in wells that produce oil without water typically produce with higher pump efficiencies and longer pump lives. As reservoirs age they naturally began producing increasing water rates as a result of underground aquifers, waterflooding or changes in the reservoir, such as wettability reversal. Pump efficiency and run life commonly decline after water production becomes significant.
Sucker rod pumps are the most common form of artificial lift for oil wells. A surface pumping unit reciprocates a rod string which is attached to a downhole rod pump. The most common surface unit is a walking beam pumping unit which converts rotary motion in a gear box to reciprocating motion in the rod string. Surface pumping units can also be hydraulically or otherwise mechanically driven. While a surface unit is normally connected to a downhole pump with a rod string, there are a number of applications where tubing replaces the rod string. Further classes of artificial lift systems have connected a motor directly to the downhole rod pump. These motors have been hydraulic or linear electrical motors. All these system reciprocate a downhole pump.
The common components of these downhole pumps are the plunger, barrel, standing valve, and traveling valve. Either the plunger or barrel is reciprocated to induce lift. A hydraulic seal formed between the plunger and barrel is fundamental to the process. The seal between the plunger and the barrel enables the pump to lift fluid. The most common sealing mechanism is a close clearance fit between the plunger and barrel as described in greater detail below. Other sealing mechanisms include plungers with ringed elements around the plunger that bridge the gap between the plunger and barrel. These elements have been made of a number of different materials. This type of sealing mechanism is typically deployed in wells that produce sand or other solids. This type of sealing mechanism does not currently appear to be effective in improving run lives in well with limited or no solids.
Close clearance seals function by forming a small gap between the plunger and the barrel which extends along the length of the plunger. The industry has defined pump “Fit” as the difference between the I.D. of the barrel and the plunger O.D. Fit is generally somewhere between two and eight thousands of an inch. The Fit allows a certain amount of fluid slippage between the plunger and barrel in order to lubricate the relative motion of the plunger and barrel. Too little fluid slippage will damage the plunger and barrel. Poor plunger lubrication is a major cause of pump failure. Excess slippage is also a major cause of pump failure, because excessive slippage results in the eroding or wearing away of the materials of the plunger and barrel. In addition, too much slippage will reduce pump efficiency and increase energy usage. Finding the optimum fluid slippage is often a well-by-well process that can change over time with changing well conditions.
Artificial lift designers typically allow fluid slippage of at least 5% of the produced fluids. However, pump failures have compelled many users to go to much higher slip rates. It is now understood that the rod pumps will slip produced water. Slipping corrosive salt water reduces plunger and barrel lives. The plunger slips fluid during specific points in the pumping cycle. By the time the pump reaches that point, the produced oil has floated away from the plunger. The water is left behind to slip around the plunger. In addition fluid slippage is inversely proportional to fluid viscosity; therefore a plunger will slip more water than oil under the same conditions.
The traveling valve is typically attached to the reciprocating plunger. However there are configurations with a moving barrel and a stationary plunger where the traveling valve is a component of the barrel. The standing valve is typically attached to the entrance of the stationary member such as the barrel, plunger, or tubing. Both these valves function as check valves; fluid only travels up into and through the pump. Therefore each stroke of the pump lifts the fluid column the length of the downhole stroke. Slippage is the volume of fluid that flows around the plunger and collects in the barrel below the plunger.
During the upstroke the traveling valve closes and the standing valve opens. The rod string lifts the plunger which lifts the fluid column. Produced fluids enter the barrel under the plunger. These fluids remain in the barrel, while waiting for the plunger to return to the bottom of the stroke, where they separate into layers of gas, oil and water. The gas is above the oil which is above the water. On the downstroke the traveling valve opens and the plunger travels through these fluid layers down to the bottom of the stroke. On the next upstroke the plunger will slip the water which is directly above the plunger. Well analysis has confirmed the preferential slippage of water over the slippage of oil.
By way of illustration, a rod-pumped well with a 144 inch stroke length and 50% water cut would have a water layer 6 feet in length standing in a stationary pump barrel. During the downstroke these layers pass through the center of the plunger and become part of the fluid column above the plunger. Therefore the 6 foot water layer would extend well above the top of a typical 3 foot plunger. In addition the gas and oil layer floats up the tubing away from the plunger. During the upstroke the traveling valve closes and the fluid is lifted, such that the pressure above the plunger is higher than below the plunger. This pressure differential induces fluid slippage around the plunger. The upstroke begins at the bottom of the stroke while the plunger is in the water layer. Therefore the plunger must slip or leak all of the water before the plunger-barrel interface is lubricated with oil. On the next stroke the plunger will pick up the water that was slipped from the previous stroke and any additional produced water. Therefore pumps will build sufficient water until they ultimately slip only water. This occurs even if produced water rates are relatively low.